Since its initial description, the pediatric eye tumor retinoblastoma 1 (Rb1) gene has been found to play a prominent role in both normal development and cancer. Consistent with its pivotal role in many cellular processes, the RB1 protein is expressed in many, if not all, tissues and is involved in the control of critical functins, such as proliferation (gene transcription, DNA replication, DNA repair, and mitosis), genome stability, differentiation, senescence, and programmed cell death, among many others. Contrasting with the simple molecular etiology of the retinoblastoma eye tumor, the mechanism underlying RB1 functional complexity and interactions, as well as its relevance in normal development and malignancies, remains elusive. While much of our current understanding of Rb1 function has resulted from molecular studies performed in cell culture models, some studies using Rb1 knockout mice have been carried out. However, the early embryonic lethality of Rb1 knockout mice poses serious limitations on undertaking any further studies in vivo, particularly in the late embryonic and postnatal stages. Although chimeric and Cre-LoxP targeting strategies have been successfully used as an alternative to further research, intrinsic limitations of these strategies (i.e., leaky promoter activity, mosaic Cre expression, lack of temporal control of transgene expression, inability to control duration of transgene activity, etc) have significantly restricted their application. To circumvent the problems associated with such approaches, we have made significant changes to an early genetic engineering protocol and developed a dominant negative (DN) tetracyclin-regulated Rb1 mouse model, in which RB1 protein can be conditionally inactivated by overexpressing a DN mutant RB1 in a reversible manner. The aims of this application are to (1) test the efficiency of transient RB1 manipulation in a tissue-specific manner and (2) evaluate changes in vestibular and auditory phenotype associated with temporally controlled RB1 downregulation. Much-needed, finely regulated and reversible Rb1 inhibition will provide the basis for the development of a wide variety of studies i an even wider number of research fields searching to shed light on the complex biochemical mechanisms underlying the various aspects of Rb1 activity. Distilling this molecular complexity into basic principles will improve our research methods and our ability to design successful therapeutic strategies.